1. Field of the Invention
The present invention relates generally to the control of bi-directional switches of triac type that automatically turn-off upon disappearance of the current through the switch. Such switches are often used to control loads powered by the electric supply mains and the switch turning-on is restarted at each half-period.
2. Discussion of the Related Art
FIG. 1 shows a first example of a conventional control circuit of a triac 1 controlling a load 2 (Q) powered by an A.C. power supply Vac (terminals P and N). In the example of FIG. 1, triac 1 has been connected in series with load 2. It should however be noted that the triacs may also be connected in parallel with the load that they control, but a parallel connection is consistent with what will be described hereafter.
In the example of FIG. 1, gate g of the triac is connected to that of the triac power electrodes which stands opposite to load 2 by a switch 3 (in this example, an NMOS transistor). Further, gate g is connected by a current-to-voltage conversion resistor R to a first terminal (+) of application of a D.C. voltage Vdc necessary to the triac control. The second terminal (−) of application of voltage Vdc is connected to terminal N of application of the A.C. voltage opposite to that to which load 2 is connected, that is, to one of the power electrodes of triac 1 and to the source of transistor 3. The gate of transistor 3 receives a control signal CTRL of logic type enabling voltage control of triac 1.
In the example of FIG. 1, triac 1 is normally on, that is, in the absence of a control signal on terminal CTRL, triac 1 is on, provided that a D.C. voltage Vdc is applied between terminals + and −. Transistor 3 is used to block the triac by preventing its automatic restarting by the short-circuiting of its gate and of its power electrode connected to terminal N.
A disadvantage of the circuit of FIG. 1 is that it generates a permanent consumption in the control circuit when triac 1 is desired to be maintained off.
Another disadvantage is that it is necessary to provide a current coming from a D.C. auxiliary power supply (voltage Vdc).
Another disadvantage of providing a normally-on triac is that in case of a malfunction of the control circuit (not shown) providing signal CTRL, triac 1 is on and load 2 remains powered. Such a situation may be dangerous and is, to say the least, not desirable.
To make triac 1 normally off in the diagram of FIG. 1, a switch 3 which is normally on and which is turned off by its control signal, which accordingly turns on triac 1, could be provided. However, the disadvantage of requiring an auxiliary power supply remains present, as well as the presence of a permanent leakage current in switch 3.
FIG. 2 shows a second conventional example of a control circuit of a triac 1 for controlling a load 2 with which it is placed in series between two terminals P and N of application of an A.C. voltage Vac. In this example, gate g of the triac is connected by a resistor R′ to power electrode 10 of the triac on the side of load 2 and is connected to terminal N (power electrode 11 of the triac opposite to terminal 2) by a switch 4 receiving a control signal CTRL. Resistor R′ starts triac 1 at each halfwave of the A.C. power supply, provided that gate g and electrode 11 are not short-circuited by switch 4.
Thus, if switch 4 is a normally-off switch, triac 1 is made normally on, which reproduces the security disadvantage discussed hereabove in relation with FIG. 1.
However, by providing a normally-on switch 4, triac 1 is blocked by default and signal CTRL causes the turning off of switch 4 when load 2 is desired to be supplied.
The circuit of FIG. 2 has the advantage of not requiring the presence of a D.C. auxiliary power supply to provide the firing of triac 1.
Further, conversely to the diagram of FIG. 1 where a leakage current is permanently present either in the triac gate, or in control MOS transistor 3, the leakage current of the example of FIG. 2 is only present in the triac gate through resistor R′.
In the diagram of FIG. 1, the use of an auxiliary power supply considerably increases the control circuit consumption. This requires either oversizing the corresponding converters, or providing batteries of sufficient capacity.
In the solution of FIG. 2, the current necessary to turn on triac 1 comes from A.C. voltage Vac, and thus generally from the electric supply mains.
A disadvantage of this solution, however, remains that triac 1 exhibits a delay upon firing at each halfwave of the A.C. power supply. This delay is due to the fact that the current running through resistor R′ must, at each halfwave of A.C. power supply Vac, become greater than the firing current of triac 1 before said triac starts conducting. Since this current also runs through load 2, the firing delay also depends on this load.
The delay is given by the following relation:
            Δ      ⁢                          ⁢      t        =                            1          ω                ·        Arc            ⁢                          ⁢              sin        ⁡                  (                                                                      I                  g                                ·                                  (                                      Z2                    +                                          R                      ′                                                        )                                            +                              V                gt                                                    Vac              cur                                )                      ,where Ig represents the gate current necessary to start triac 1, Z2 represents the impedance of load 2, Vgt represents the voltage of gate g of the triac, Vaccur represents the maximum value of A.C. voltage Vac, and ω represents the pulse of voltage Vac.
A firing delay of the triac at each halfwave of the power supply is prejudicial since this causes unwanted current and/or voltage peaks.
FIGS. 3A and 3B are timing diagrams illustrating the triac firing delay phenomenon on a resistive load. FIG. 3A shows an example of the shape of A.C. supply voltage Vac and of voltage V1 across the triac. FIG. 3B illustrates the shape of current I through load 2. For simplification, leakage currents and the voltage drops that they generate are here neglected.
Triac 1 is initially assumed to be off. Accordingly, current I is zero and voltage V1 across triac 1 corresponds to A.C. voltage Vac. The state of control signal CTRL is assumed to be reversed at a time t1 to turn triac 1 on. If time t1 occurs sufficiently late in a period of voltage Vac, the triac starts at this time t1 and a non-zero current I then flows through load Q. Since the presence of a resistive load has been assumed, the shape of current I is synchronous with the shape of voltage Vac. At the next zero crossing of the current flowing through the triac, said triac turns off. The end of the delay Δt following the beginning of the halfwave must then be awaited before the triac firing occurs. The higher the resistance of load Q, the greater the firing delay will be before the firing current of the triac is reached.
A new reversal of current CTRL is assumed at time t2 to turn off the circuit by the turning-off of triac 1. In the example of FIG. 2, this amounts to turning on switch 4. The triac is however only blocked at the next zero crossing of voltage Vac by the canceling of the current flowing therethrough. From this zero crossing on, voltage V1 becomes approximately equal again to voltage Vac.
It should be noted that for such a circuit to operate, resistance R′ must be very large as compared to the resistance of load 2, unless which the leakage current when switch 4 is on would be too high, which would amount to turning on load 2.
FIGS. 4A and 4B illustrate the operation of a conventional triac control circuit of the type shown in FIG. 2 on an essentially inductive load Q.
Here again, a turning-off of switch 4 is assumed at a time t1 to fire the triac. Since the load is inductive, current I through the load increases from 0 (instead of the peak on a resistive load). The inductive load delays the triac turning-off upon zero crossing of voltage Vac. However, this causes a voltage peak in V1 at each zero crossing of current I since voltage Vac across the circuit is then different from zero.
At a time t2 where control signal CTRL reverses to turn on switch 4, current I does not immediately disappear, but awaits the end of its halfwave. At that time, voltage V1 abruptly joins voltage Vac.
The significance of voltage peaks V1 depends on the value of the inductance of load 2, which conditions the phase-shift between current I in the load and A.C. voltage Vac.